The invention relates to the field of elastomeric articles used in the medical field. In particular, the invention relates to improvements to the process of making elastomeric polyisoprene articles for medical applications.
The manufacturing process for producing elastomeric articles from natural or synthetic rubber latex involves a curing step during which cross-linking or vulcanization through sulfur groups occurs between the polymer units. Conventional processes for making elastomeric articles from natural or synthetic latex typically involve preparing a latex dispersion or emulsion, dipping a former in the shape of the article to be manufactured into the latex and curing the latex while on the former. Desirable properties of certain elastomeric articles such as tensile strength are substantially affected by the cross-linking and curing stages of the manufacturing process.
The use of vulcanizing or sulfur cross-linking accelerator compounds in the manufacture of rubber articles is well-known. Conventional vulcanization accelerators include dithiocarbamates, thiazoles, guanidines, thioureas, amines, disulfides, thiurams, xanthates and sulfenamides. The use of vulcanization accelerators in the manufacture of polyisoprene rubber is disclosed in D""Sidocky et al., U.S. Pat. No. 5,744,552 and Rauchfuss et al., U.S. Pat. No. 6,114,469. Certain fields in which elastomeric articles are needed, such as the medical field, utilize specific types of equipment and processing techniques which accommodate the specific performance and regulatory requirements of the particular article produced.
The use of natural rubber latex in the manufacture of certain articles such as medical gloves has been associated with disadvantageous properties, such as allergic reactions believed by some to be caused by natural proteins or allergens present within the natural rubber latex and the final product. Of increasing interest in the medical field, particularly in the field of gloves, are synthetic elastomeric products and manufacturing processes which altogether reduce, or altogether avoid, the likelihood of potential adverse reactions of the user or wearer.
Synthetic elastomeric polyisoprene articles such as gloves are known and are of interest in the art as an alternative to the use of natural latex. Commercially available synthetic gloves include those elastomers composed of polychloroprene (neoprene), carboxylated acrylonitrile butadiene (nitrile), styrene-isoprene-styrene/styrene-ethylene-butylene-styrene block co-polymers, polyurethane, and polyisoprene. Polyisoprene is one of the most preferred polymers due to its chemical similarity to natural rubber as well as its physical properties such as feel, softness, modulus, elongation and tensile strength. One such polyisoprene glove is commercially available from Maxxim Medical (Clearwater, Fla.).
A majority of glove manufacturing processes are water-based dipping systems. It is known that solvent-based systems are possible for polyisoprene, although such systems are poorly suited for the manufacture and molding of elastomeric articles for medical applications. One difficulty in the field of gloves, for example, is the design of processes and materials which will produce a thin elastomeric article having desirable properties such as high tensile strength. Another disadvantage of solvent-based systems is solvent toxicity. Process and materials which would obviate or reduce the need for the use of toxic solvents while at the same time yielding a product having desirable properties for medical applications are thus still being explored.
Accordingly, there exists a need in the medical device field for improved manufacturing processes for making synthetic elastomeric articles. Especially desirable would be processes which can produce polyisoprene articles, such as surgical gloves, which possess the desirable properties found in the natural rubber counterpart, while at the same time permitting economical and cost-effective manufacturing.
Applicants have discovered a three-part accelerator composition for sulfur cross-linkable polyisoprene latex which can be used with latex in a process for making elastomeric articles having the desirable properties (e.g., tensile strength) similar to that of natural rubber but without the presence of natural rubber latex proteins and allergens. Another advantage is that the accelerator system is suitable for medical applications where thin molded elastomeric articles are required, such as gloves. Furthermore, the accelerator composition and process of the invention permits the use of a solvent-free, water-based process system, as opposed to a solvent-based process system. The resultant article has properties similar to those produced using the solvent-based system. Accordingly, the use of solvents can be reduced or avoided and solvent toxicity can likewise be avoided using the invention.
Another advantage of the invention is that conventional manufacturing equipment and most readily-available materials can be used in accordance with the invention to make the synthetic polyisoprene glove without the need for new or costly additional materials or equipment. Further, no complicated new process steps are required by the invention and the invention can be readily incorporated into existing glove making processes and systems.
Another aspect of the invention is that the compounded (or ready to use) polyisoprene latex composition formulated in accordance with the invention exhibits prolonged storage stability. For example, the pre-cure storage stability of the compounded polyisoprene latex composition (i.e., the time period prior to the use of the compounded polyisoprene latex composition in the dipping and curing stages) can extend up to about 8 days, in contrast to the typical current 3 to 5 day time period. By extending storage life of the latex, the amount of wasted latex can be significantly reduced and greater flexibility in scheduling manufacturing processes is permitted.
Yet another advantage is that the process of the invention allows for significantly reduced pre-cure process parameters (lower temperature and shorter time periods than conventionally used) and lower dipping temperatures in the manufacturing process. Accordingly, significant cost and resource advantages are provided over conventional manufacturing practices.
The invention provides for a process of making a synthetic elastomeric polyisoprene article comprising the steps of: a) preparing a compounded polyisoprene latex composition containing an accelerator composition containing a dithiocarbamate, a thiazole and a guanidine compound; b) dipping a former into said compounded polyisoprene latex composition; and c) curing said compounded polyisoprene composition on said former. Additionally, the initial pre-cure processing (i.e., prior to storage and article manufacture) can be performed at temperatures of less than 35xc2x0 C. and in time periods as short as ranging from about 90 minutes (1.5 hours) to about 150 minutes (2.5 hours), preferably about 120 minutes (2.0 hours). The compounded polyisoprene latex composition can be stored for periods up to about 8 days at ambient temperatures (ranging from about 15xc2x0 C. to about 20xc2x0 C.). Lower temperatures can be used for the latex dipping step as well.
The invention also provides for a synthetic elastomeric polyisoprene article made by a process comprising the steps of: a) preparing a compounded polyisoprene latex composition comprising an accelerator composition comprising a dithiocarbamate, a thiazole and a guanidine compound; b) pre-curing said compounded polyisoprene latex composition c) dipping a former into said compounded polyisoprene latex composition; and d) curing said compounded polyisoprene composition on said former. Elastomeric articles made by the process of the invention can exhibit tensile strengths of over 3000 psi (as measured in accordance with ASTM D412) even after as much as 7 days of latex storage prior to use in the article manufacturing process.
The invention further provides for a synthetic polyisoprene latex composition comprising:
polyisoprene latex;
a dithiocarbamate compound;
a thiazole compound; and
a guanidine compound.
The invention also provides for an accelerator composition for use in making elastomeric polyisoprene articles consisting essentially of:
a dithiocarbamate compound;
a thiazole compound;
a guanidine compound;
wherein the phr (parts per hundred) dry weight ratio of each of the dithiocarbamate; thiazole; and guanidine ranges from about 0.50 to about 1.00 per 100.0 parts polyisoprene.
In a preferred embodiment, the accelerator composition comprises zinc diethyithiocarbamate (ZDEC), zinc 2-mercaptobenzothiazole (ZMBT), and diphenyl guanidine (DPG) and used in conjunction with a stabilizer. Preferably, the stabilizer is an alkali metal caseinate salt, such as sodium caseinate.
The accelerator composition of the invention can be used in conjunction with conventional equipment and materials otherwise known to be used in the manufacture of elastomeric articles composed of polyisoprene. In general, the process begins with the preparation of the compounded polyisoprene latex composition. The synthetic polyisoprene latex is combined with the accelerator composition, a stabilizer, and additional ingredients to prepare the polyisoprene latex composition in accordance with the invention. The function of the accelerator is to increase the rate of vulcanization, or the cross-linking of polyisoprene to enhance the curing properties of the latex during the curing stages of the process. Prior to the dipping and curing steps, the compounded latex including the accelerator composition can be used immediately or stored for a period of time prior to its employment in the dipping process.
When the compounded polyisoprene latex composition is ready for use or following storage, a former in the overall shape of the article to be manufactured is first dipped into a coagulant composition to form a coagulant layer directly on the former. Next, the coagulant-coated former is dried and then dipped into the compounded polyisoprene latex composition.
The latex-covered former is then subjected to the curing step. The latex is cured directly onto the former at elevated temperatures thereby producing an article in the shape of the former. Further steps are typically performed as well, such as leaching with water, beading the cuff, and the like. These techniques are well-known in the art. Additional post-treatment processes and techniques steps are often performed as well, such as lubrication and coating, halogenation (e.g., chlorination), and sterilization.
A variety of elastomeric articles can be made in accordance with the invention. Such elastomeric articles include, but are not limited to, medical gloves, condoms, probe covers (e.g., for ultrasonic or transducer probes), dental dams, finger cots, catheters, and the like. As the invention provides numerous advantages and benefits in a number of ways, any form of elastomeric article which can be composed of polyisoprene can benefit from the use of the invention.
Polyisoprene latex is the major component of the pre-cure latex composition. Suitable polyisoprene latex which can be used is readily available and can be obtained from a number of commercial sources, including but not limited to, Kraton(trademark) Corporation, Houston, Tex.; Shell International Corporation, Houston, Tex.; Apex Medical Technologies, Inc. San Diego, Calif.; and Aqualast(trademark) E0501 available from Lord Corporation, Erie, Pa. In addition to polyisoprene, polyisoprene co-polymers and polyisoprene blends can be used as well. Polyisoprene co-polymers which can be used include any co-polymer having an isoprene monomer unit and having sufficiently similar chemical structural and properties of polyisoprene to exhibit the desirable properties of the polyisoprene product when combined with the accelerator composition and made according to the process of the invention. Suitable polyisoprene blends can include, but are not limited to: natural rubber latex; polydiene and its co-polymers, such as polybutadiene; substituted polydiene, such as polychloroprene; thermoplastic materials, such as polyurethane; and the like.
The accelerator composition of the invention comprises at least one dithiocarbamate, at least one thiazole, and at least one guanidine compound. Preferably, the dithiocarbamate compound for use with the invention is zinc diethyldithiocarbamate, also known as ZDEC or ZDC. Suitable ZDEC which can be used includes Bostex(trademark) 561 (commercially available from Akron Dispersions, Akron, Ohio). The preferred thiazole compound for use in the invention is zinc 2-mercaptobenzothiazole, also known as zinc dimercaptobenzothiazole or ZMBT. Suitable ZMBT which can be used includes Bostex(trademark) 482A (commercially available from Akron Dispersions, Akron, Ohio). In a preferred embodiment, the guanidine compound used in the accelerator composition is diphenyl guanidine, also known as DPG. Suitable DPG which can be used includes Bostex(trademark) 417 (commercially available from Akron Dispersions, Akron, Ohio).
Other dithiocarbamate, thiazole and guanidine derivatives can also be use in accordance with the invention, provided each is chemically compatible with, i.e., does not substantially interfere with the functionality of, the remaining two accelerator compounds used. Dithiocarbamate derivatives which can also be used include zinc dimethyldithiocarbamate (ZMD), sodium dimethyldithiocarbamate (SMD), bismuth dimethyldithiocarbamate (BMD), calcium dimethyldithiocarbamate (CAMD), copper dimethyldithiocarbamate (CMD), lead dimethyldithiocarbamate (LMD), selenium dimethyldithiocarbamate (SEMD), sodium diethyldithiocarbamate (SDC), ammonium diethyldithiocarbamate (ADC), copper diethyldithiocarbamate (CDC), lead diethyldithiocarbamate (LDC), selenium diethyldithiocarbamate (SEDC), tellurium diethyldithiocarbamate (TEDC), zinc dibutyldithiocarbamate (ZBUD), sodium dibutyldithiocarbamate (SBUD), dibutyl ammonium dibutyldithiocarbamate (DBUD), zinc dibenzyldithiocarbamate (ZBD), zinc methylphenyl dithiocarbamate (ZMPD), zinc ethylphenyl dithiocarbamate (ZEPD), zinc pentamethylene dithiocarbamate (ZPD), calcium pentamethylene dithiocarbamate (CDPD), lead pentamethylene dithiocarbamate (LPD), sodium pentamethylene dithiocarbamate (SPD), piperidine pentamethylene dithiocarbamate (PPD), and zinc lopetidene dithiocarbamate (ZLD).
Other thiazole derivatives which can be used include 2-mercaptobenzothiazole (MBT), copper dimercaptobenzothiazole (CMBT), benzothiazyl disulphide (MBTS), and 2-(2xe2x80x2,4xe2x80x2-dinitrophenylthio) benzothiazole (DMBT).
Other guanidine derivatives which can be used include diphenyl guanidine acetate (DPGA), diphenyl guanidine oxalate (DPGO), diphenyl guanidine phthalate (DPGP), di-o-tolyl guanidine (DOTG), phenyl-o-tolyl guanidine (POTG), and triphenyl guanidine (TPG).
The proportions and ratios of the ingredients of the accelerator composition can vary somewhat provided all three of the ingredients, i.e., dithiocarbamate, thiazole and guanidine compounds, are present. With respect to the preferred accelerator ingredients, each of the accelerator compounds zinc diethyldithiocarbamate (ZDEC), zinc 2-mercaptobenzothiazole (ZMBT) and diphenyl guanidine (DPG) can be present in an individual amount ranging from about 0.50 phr (parts by weight per 100 parts by weight of rubber) to about 1.00 phr dry weight per 100 parts polyisoprene. In other words, the accelerator compositions of the invention comprise ZDEC:ZMBT:DPG phr dry weight ratios ranging respectively from about 0.50:0.50:0.50 phr to about 1.00:1.00:1.00 phr.
In a preferred embodiment, a stabilizer is used in conjunction with the accelerator composition. Any stabilizer known in the art useful in curable latex systems can be used provided it is chemically compatible with the other ingredients and provides the desired function, i.e., prolongs stabilization of the pre-cure compounded polyisoprene latex. A variety of stabilizers can be used, including but not limited to, milk protein salts, anionic surfactants such as sodium lauryl sulfates, and sorbitan fatty acid esters.
Milk protein salts are preferred for use as the stabilizer. In particular, alkali metal caseinate salts are preferred. Alkali metal caseinate salts which can be used in accordance with the invention include, but are not limited to, sodium caseinate, potassium caseinate, manganese caseinate and zinc caseinate, and combinations thereof. Most preferred for use as the stabilizer is sodium caseinate (commercially available from Technical Industries, Inc., Peacedale, R.I.).
Anionic surfactants which can be used as stabilizers for the invention include Rhodopex(copyright) ES (a composition having a sodium lauryl (3) sulfate active available from Rhodia, Cranbury, N.J.) and Rhodacal(copyright) DS-10 (a composition having a branched sodium dodecylbenzene active available from Rhodia, Cranbury, N.J.). Sorbitan fatty acid ester surfactants which can be used as stabilizers in the invention include polyoxyethylene sorbitan fatty acid esters such as Tween(copyright) 80 (a polysorbate available from ICI Americas, Inc., Wilmington, Del.).
The amount of stabilizer present in the pre-cure polyisoprene latex composition is preferably ranges from about 0.50 phr dry weight to about 1.00 phr dry weight (per 100.00 parts dry weight polyisoprene). Preferably, the amount of stabilizer is present in an amount of about 0.75 phr dry weight.
In addition to the polyisoprene, accelerator composition and stabilizer, additional ingredients which enhance or facilitate the manufacturing process can be included in the compounded polyisoprene latex composition as well. The compounded polyisoprene latex composition can also include catalysts (or accelerator initiators) such as alkali earth metal oxides and methyl oxides, preferably zinc oxide (ZnO) (commercially available from Maxxim Medical, Eaton, Ohio); curing (or cross-linking) agents such as elemental Sulfur (e.g., Bostex(trademark) 378 commercially available from Akron Dispersion, Akron, Ohio), organic sulfides or other sulfur donor compounds; and anti-oxidants, such as Wingstay(trademark) (e.g., butylated reaction product of p-cresol and dicyclopentadiene (DCPD) such as Bostex(trademark) 24 available from Akron Dispersion, Akron, Ohio).
The compounded polyisoprene latex composition in accordance with the invention can be prepared using the following general procedure:
Polyisoprene latex (typically 60% solids) and the stabilizer (e.g., sodium caseinate) are combined at ambient temperature (about 20xc2x0 to about 25xc2x0 C.). After mixing for a period of time, the mixture is then diluted to 40% solids in water. Wingstay L is then added and the mixture is stirred for approximately 15 minutes. At this point, the pH can be adjusted to a range of about 8.5 to 9.0. Zinc oxide is added, followed by the sulfur and accelerator compounds. Preferred accelerator compounds are ZDEC, ZMBT and DPG and are added in ratios ranging from 0.50:0.50:0.50 phr to 1.00:1.00:1.00 phr dry weight per 100.0 parts polyisoprene. The mixture is then heated to a temperature within a range of about 20xc2x0 C. to about 40xc2x0 C., preferably from about 25xc2x0 C. to about 30xc2x0 C., while continuously stirring for a time period ranging from about 1.5 hours to about 2.5 hours, preferably about 2 hours, using a magnetic stirrer and heating plate.
The mixture is then cooled to a temperature ranging of less than about 25xc2x0 C., typically ranging from about 15xc2x0 C. to about 20xc2x0 C. The compounded latex is preferably stored at ambient temperatures ranging from about 15xc2x0 to about 20xc2x0 C. At these temperatures, the compounded polyisoprene latex composition can be stored for periods lasting up to about 8 days prior to its use in the dipping and curing process.
Initially, the pH of the compounded polyisoprene latex can be adjusted to a pH of approximately 10. A glove former is pre-heated in an oven to a temperature of about 70xc2x0 C. and then dipped in a pre-prepared coagulant composition at a temperature of about 55xc2x0 C. for a period of time and then removed therefrom. Next, the coagulant-coated former is placed in a drying oven at 70xc2x0 C. for a time sufficient to dry the coagulant, typically about 5 minutes.
The coagulant-coated former is removed from the oven and dipped into the compounded polyisoprene latex at ambient temperature, or a temperature ranging from about 20xc2x0 C. to about 25xc2x0 C. The coated former is removed and placed in oven at a temperature of about 70xc2x0 C. for about 1 minute. The glove and former are removed from oven and placed into water leaching tank having a temperature of about 65xc2x0 C. for about 5 minutes. The glove and former are removed from the leaching tank and placed dried at about 70xc2x0 C. for a period sufficient to dry the glove, typically about 5 minutes. This is the end of the first curing stage.
At the second curing stage, the glove and former are placed in an oven heated to a temperature of about 120xc2x0 C. for about 20 minutes. The glove and former are removed and cooled to ambient temperature. Finally, the glove is stripped from the former.
The gloves can be further treated in accordance with the particular needs, such as using lubrication, coating, halogenation, and sterilization techniques, all of which are conventional. Other conventional steps can be incorporated into the general process as well.
When prepared in accordance with the invention, elastomeric articles such as gloves exhibit the following physical properties: tensile strength of greater than about 3000 psi, elongation of greater than about 750% at break, and a tensile modulus of less than about 300 psi at 300% elongation as measured in accordance with ASTM D412.
Other elastomeric polyisoprene articles can be prepared using processes similar to those described herein, in combination with conventional equipment and techniques readily available in the art. For example, an elastomeric article in the form of condom can be prepared using a condom former.
The following example further illustrates the advantages of the invention and should not be construed as limiting the invention to the embodiments depicted therein.